If a molten metal temperature can be measured accurately and continuously in a furnace for refining a molten metal, such as a converter or an AOD, and the temperature can be acquired as operation information, it would be extremely advantageous for improving efficiency of refining, quality and operation yield and for reducing various original units. To efficiently conduct refining, it is very important to continuously determine the molten metal temperature and to control the temperature to a temperature shift pattern determined in advance for each type of steel. Therefore, various methods for measuring the molten metal temperature inside the refining furnace have been attempted and improved in the past. At first, the refining furnace was tilted and an operator manually measured the molten metal temperature by using a measurement probe of a consumption type thermo-couple equipped with a protection pipe. However, this method was not free from the problems of a lack of safety due to the tilt operation and the drop of productivity due to the measurement time. Therefore, a sub-lance capable of improving an operation factor and productivity was developed and introduced. This method made it possible to automatically fit a measurement probe of a consumption type thermo-couple equipped with a protection pipe to the distal end of a water cooling lance and to conduct a remote and automatic temperature measurement.
Though capable of accurately measuring the temperature, these measurement probe systems would likely not continuously measure the molten metal temperature during refining and would likely not either execute fine refining control because they were intermittent temperature measurement systems. Because the measurement probe for the temperature measurement was consumed, the cost was high, too.
In contrast, attempts to continuously measure the temperature of the molten metal have been made in the past. An apparatus that guides thermal radiation light of the molten metal facing a nozzle distal end to a radiation thermometer through an optical fiber while preventing invasion of the molten metal into a nozzle by pressure feeding an inert gas into the nozzle penetrating through a molten metal vessel is known (for example, as described in Japanese Patent Publication Nos. 61-91529, 62-52423 and 8-15040). Further, U.S. Pat. No. 6,172,367 (corresponding to Japanese Patent Publication No. 2000-502183) describes a molten metal measurement method of an optical fiber system. Though this method can conduct continuous measurement, it is not free from the following problems. When the center of the field of the optical fiber deviates from the nozzle center or when the optical axis inclines from the nozzle center axis, the molten metal in the proximity of the nozzle distal end is likely to be solidified. When the solidified molten metal closes a part of the fiber field, radiation energy the fiber receives decreases, so that the apparent temperature is observed at a lower level. It is not possible at present to judge from the output of the radiation thermometer whether the field is closed or the temperature has actually dropped. Therefore, this method involves the problem of reliability of the measurement value. All these problems result from the fact that the measurement is so-called “point measurement” because the optical fiber is used. Incidentally, Japanese Patent Publication No. 8-15040 describes a method that feeds the optical fiber towards the molten metal to prevent the occurrence of deviation of the optical axis and brings the distal end of the optical fiber into contact with the molten metal. However, using method, the cost becomes high due to the consumption of the optical fiber.
Another apparatus (e.g., as described in Japanese Patent Publication No. 11-142246) uses a thermal radiation light emitted from the molten metal are inputted into an imaging device (such as a CCD camera) through an image fiber and measures the temperature of the molten metal from the highest luminance value on an imaging screen. Because the image fiber system used in the invention can measure the image, it can drastically improve the problems described above. The image fiber is fabricated by finely bonding more than 15,000 optical fibers and bundling them into a diameter of about 4 mm. A condenser lens having a focal length of near infinity is fitted to the distal end of the image fiber and the image in front is projected to a light reception end of the image fiber. A projection image (optical image) is as such transmitted to a light outgoing end of the image fiber. In other words, the image fiber has an optical image transmission function and transmits the optical image in front from the light reception end to the light outgoing end. The imaging device images the optical image at the light outgoing end and generates image signals. In this way, image measurement, and image analysis on the basis of the former, become possible. Consequently, it may become possible to recognize strangulation of the field at a tuyere distal end, by the molten metal, that has not been possible by use of the optical fiber, and to measure the correct temperature. Even when the optical image moves the field, due to deviation of the optical axis to a certain extent, the temperature can be measured correctly without any problem by conducting image processing. The apparatus according to the present invention has the advantage that it can continuously measure the molten metal temperature. Because two-dimensional observation is made by use of the image fiber and the molten metal is automatically extracted by image processing, deviation of the optical axis to a certain extent does not cause any problem.
In the apparatuses using the image fiber described above, however, the fiber should be fitted and removed at the time of the exchange of the converter, the AOD furnace, etc., even when conformity of the optical axes is once secured, and the optical axes are likely to greatly deviate at this time. The image fiber, that is particularly expensive, is likely to undergo thermal damage due to excessive, heat and is sometimes broken when it is fitted and removed. Furthermore, because cleaning of the pressure-resistant window glass partitioning the nozzle and the image fiber and measurement of the melting loss amount of the nozzle distal end are carried out during the operation at high temperature, they must be carried out easily and quickly.
These features are described in Japanese Patent Publication No. 2001-83013. In this publication, a tuyere portion for introducing thermal radiation light from the molten metal, a purge gas introduction portion, a pressure-resistant window glass holding portion, an image fiber centering portion and an image fiber protection tube are detachably connected to one another, and the image fiber centering portion is constituted by concave and convex portions that can be fitted and keep close contact with one another so as to facilitate centering. However, according to the construction of this centering portion, each part constituted by the concave and convex portions employs fitting and close contact and cannot easily adjust the deviation of the optical axis. Because coupling with the nozzle uses a flange structure, fitting/removal needs a long time and a quick operation in the high temperature atmosphere cannot be carried out easily. Because two nozzle gas introduction portions are generally necessary in the case of a double pipe nozzle in comparison with the single pipe nozzle in this system, the distance between the distal end of the image fiber and the distal end of the nozzle as a thermal radiation light inlet becomes great and the adjustment of the optical axes can become difficult.
A method for continuously measuring the molten metal temperature is described, for example, in Japanese Patent Publication Nos. 60-129628 and 61-17919. In these references, a radiation thermometer is fitted to the rear end of a temperature measurement nozzle penetrating through a refractory of a converter or a ladle and the molten metal temperature is measured from thermal radiation light of the molten metal facing the nozzle distal end while a gas is jetted from the temperature measurement nozzle to the molten metal. In this method, however, the gas jetted from the nozzle always cools the interface between the refractory and the molten metal in the proximity of the nozzle distal end and solidified steel called a “mushroom” (hereinafter called “solidified metal”) is created. The growth of this solidified metal frequently closes the nozzle. As a result, the radiation thermometer measures the solidified metal having a lower temperature than the molten metal and a great error occurs in the measurement value.
To remove the solidified metal, it may be possible to mix oxygen with the gas to be blown and to melt the solidified metal by heat of the oxidation reaction. It has been found that according to this method, however, the melting loss of the nozzle drastically proceeds due to the rise of the molten metal temperature and the measurement cannot be made. To suppress the growth of the solidified metal, on the other hand, it may be possible to employ a method that reduces the flow rate of an inert gas and minimizes cooling of the molten metal interface. When the flow rate is insufficient, critical problems develop in that the molten metal enters the nozzle and not only the light receiver is broken but also the molten metal flows outside.
A method for preventing adhesion of the solidified metal is described in Japanese Patent Publication No. 11-281485, in which an inner diameter of a temperature measurement nozzle is set to 3 to 5 mm and stipulates the flow rate of an inert gas to be jetted from the temperature measurement nozzle to the molten metal to a range in which the solidified metal does not grow at the nozzle distal end and also does not enter the nozzle.
According to this method, however, the nozzle diameter is as small as 3 to 5 mm and the thickness of the refractory through which the nozzle penetrates is as large as about 1 m. Therefore, when bending occurs in the nozzle due to thermal deformation of the refractory, the problem occurs in that the field capable of being observed cannot be secured sufficiently. The temperature change of the molten metal is great inside the refining furnace due to the unbalance between exothermy resulting from blowing of oxygen and heat removal resulting from the addition of a cooling material and clogging of the nozzle distal end by the solidified metal cannot be completely prevented.
A refractory brick which the nozzle penetrates delicately can change its position, with the passage of the time, due to thermal expansion resulting from the high temperature, while the molten metal continuous temperature measurement of the present system is carried out. Thus, the nozzle itself may be bent, and the field can become narrow. To cope with such problems, it is preferable to delicately move the light reception portion at the distal end of the image fiber in such a fashion that its optical axis is coincident with the bending direction of the nozzle. Such fine adjustment of the light reception portion at the distal end of the image fiber is difficult if not impossible to attain by the prior art technologies and a new counter-measure is needed.
The wall of the vessel accommodating the molten metal reaches a high temperature due to heat transfer from the molten metal. Therefore, the nozzle and the image fiber connection device reach the high temperature, too, due to both of thermal conduction and radiation heat. When the furnace is tilted and the image fiber connection device is exposed to a hood inner surface at the upper part of the furnace, the device receives heat radiation from a red hot base metal adhering to the hood inner surface. Accordingly, the cooling system that protects the image fiber and the imaging device from these thermal influences, and the control method of the cooling system, should be preferably provided.
Because thermal radiation light from the molten metal is irradiated to the inner surface of the temperature measurement nozzle and to the inner surface of the connection portion from the nozzle to the image fiber, inner surface reflected light exists in the proximity of direct light from the molten metal and the temperature measurement apparatus sometimes fails to measure the correct temperature. A sufficient effect cannot be obtained to solve this problem even when a counter-measure, that brings, as much as possible, the center axis of the temperature measurement nozzle into conformity with the optical axis of the image fiber, is employed.
Because the inert gas is pressure fed into the temperature measurement nozzle to prevent invasion of the molten metal, the field of the image fiber becomes narrow or the nozzle is clogged when the molten metal in the proximity of the distal end of the nozzle is solidified and cuts off thermal radiation light from the molten metal.
A method that switches the inert gas to oxygen gas and melts the base metal at the distal end of the nozzle at the time of clogging of the temperature measurement nozzle has been previously described (for example, in Japanese Patent Publication No. 60-231141 and CAMP-ISIJ Vol. 2(1989), p. 216). However, when this method is excessively executed, the melting loss of the nozzle becomes remarkably great and when the timing of the execution of this method is not proper, the solidified metal cannot be melt and flow away even when the oxygen gas is blown. These proposals do not concretely disclose the execution method and the melt-flowing of the base metal by oxygen cannot be utilized effectively.
Japanese Patent Publication No. 60-129628 describes a method that mixes a suitable amount of oxygen with the inert gas to be blown from the temperature measurement nozzle and measures the temperature. However, the interface temperature between the blown gas and the molten metal greatly changes depending on the degree of mixing of oxygen in the blown gas. Further, because fine adjustment of the mixing proportion of the gas is difficult, it is difficult to conduct the molten metal temperature measurement with high accuracy.
Japanese Patent Publication No. 11-326061 describes a method that does not use a dedicated temperature measurement nozzle, generally flows the oxygen gas mixed with nitrogen as the blowing gas to suppress the growth of the solidified metal of the nozzle, flows nitrogen through the nozzle for the temperature measurement and switches the gas to the blowing oxygen gas after the temperature measurement is completed. This method is a so-called “batch temperature measurement method” that measures the temperature at certain points, and cannot accomplish continuous temperature measurement of the molten metal.
Each of the references cited herein are incorporated herein by reference in their entireties.
As described above, a large number of problems have yet been left unsolved and quick solution of these problems has been urgently required.